Astrophysics Absolute Dimensions of Eclipsing Binaries
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University of Warwick institutional repository: http://go.warwick.ac.uk/wrap This paper is made available online in accordance with publisher policies. Please scroll down to view the document itself. Please refer to the repository record for this item and our policy information available from the repository home page for further information. To see the final version of this paper please visit the publisher’s website. Access to the published version may require a subscription. Author(s): J. Southworth, J. V. Clausen Article Title: Absolute dimensions of eclipsing binaries - XXIV. The Be star system DW Carinae, a member of the open cluster Collinder 228 Year of publication: 2007 Link to published article: http://dx.doi.org/10.1051/0004-6361:20065614 Publisher statement: © ESO 2007. Southworth, J. and Clausen, J. V. (2007). Absolute dimensions of eclipsing binaries - XXIV. The Be star system DW Carinae, a member of the open cluster Collinder 228. Astronomy & Astrophysics, Vol.461 (3), pp.1077-1093 A&A 461, 1077–1093 (2007) Astronomy DOI: 10.1051/0004-6361:20065614 & c ESO 2007 Astrophysics Absolute dimensions of eclipsing binaries XXIV. The Be star system DW Carinae, a member of the open cluster Collinder 228, J. Southworth1,2 and J. V. Clausen1 1 Niels Bohr Institute, Copenhagen University, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark 2 Department of Physics, University of Warwick, Coventry, CV4 7AL, UK e-mail: [email protected] (JS), [email protected] (JVC) Received 16 May 2006 / Accepted 11 October 2006 ABSTRACT Context. The study of detached eclipsing binaries which are members of stellar clusters is is a powerful way of determining the properties of the cluster and of constraining the physical ingredients of theoretical stellar evolutionary models. Aims. DW Carinae is a close but detached early B-type eclipsing binary in the young open cluster Collinder 228. We have measured accurate physical properties of the components of DW Car (masses and radii to 1%, effective temperatures to 0.02 dex) and used these to derive the age, metallicity and distance of Collinder 228. Methods. The rotational velocities of both components of DW Car are high, so we have investigated the performance of double- Gaussian fitting, one- and two-dimensional cross-correlation and spectral disentangling for deriving spectroscopic radial velocites in the presence of strong line blending. Gaussian and cross-correlation analyses require substantial corrections for the effects of line blending, which are only partially successful for cross-correlation. Spectral disentangling is to be preferred because it does not assume anything about the shapes of spectral lines, and is not significantly affected by blending. However, it suffers from a proliferation of local minima in the least-squares fit. We show that the most reliable radial velocities are obtained using spectral disentangling constrained by the results of Gaussian fitting. Complete Strömgren uvby light curves have been obtained and accurate radii have been measured from them by modelling the light curves using the Wilson-Devinney program. This procedure also suffers from the presence of many local minima in parameter space, so we have constrained the solution using an accurate spectroscopic light ratio. The effective temperatures and reddening of the system have been found from Strömgren photometric calibrations. Results. The mass and radius of DW Car A are MA = 11.34 ± 0.12 M and RA = 4.558 ± 0.045 R. The values for DW Car B are MB = 10.63 ± 0.14 M and RB = 4.297 ± 0.055 R. Strömgren photometric calibrations give effective temperatures of Teff A = 27 900 ± 1000 K and TeffB = 26 500 ± 1000 K, and a reddening of Eb−y = 0.18 ± 0.02, where the quoted uncertainties include a contribution from the intrinsic uncertainty of the calibrations. The membership of DW Car in Cr 228 allows us to measure the distance, age and chemical composition of the cluster. We have used empirical bolometric corrections to calculate a distance modulus of 12.24 ± 0.12 mag for DW Car, which is in agreement with, and more accurate than, literature values. A comparison between the properties of DW Car and the predictions of recent theoretical evolutionary models is undertaken in the mass-radius and mass-Teff diagrams. The model predictions match the measured properties of DW Car for an age of about 6 Myr and a fractional metal abundance of Z ≈ 0.01. Key words. stars: fundamental parameters – stars: binaries: eclipsing – stars: emission-line, Be – stars: distances – stars: individual: DW Carinae – open clusters and associations: individual: Collinder 228 1. Introduction radial velocity curves (e.g., Southworth et al. 2005b; Torres et al. 1997). The effective temperatures of the two stars can be found The study of detached eclipsing binary star systems (dEBs) is by spectral analysis or photometric calibrations, allowing the lu- one of the few ways in which the absolute properties of stars can minosities and distance of the stars to be calculated directly (e.g., be measured directly and accurately (Andersen 1991). These ob- Southworth et al. 2005a; Clausen & Giménez 1991). jects are therefore of fundamental importance to the understand- ing of the characteristics of single stars. The masses and radii of One of the most important uses of the physical properties of the component stars in dEBs can be determined to accuracies of dEBs is as a check of the predictions of theoretical models of better than 1% from the analysis of light curves and double-lined stellar evolution. The two components of a dEB have the same age and chemical composition, as they were born together, but in general different masses, radii and luminosities. Theoretical Based on observations carried out with the Strömgren Automatic Telescope (SAT) and the FEROS spectrograph at ESO, La Silla, Chile models must therefore be able to match their accurately-known (62.H-0319, 62.L-0284, 63.H-0080, 64.L-0031). properties for one age and chemical composition. A good match Full list of the radial velocity measurements is only available in is often achieved (e.g., Southworth et al. 2004b) for two rea- electronic form at the CDS via anonymous ftp to sons. Firstly, the models are in general fairly reliable, and the ef- cdsarc.u-strasbg.fr (130.79.128.5) or via fects of improved input physics lead to only minor adjustments http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/461/1077 in the evolutionary phases for which we are able to study dEBs. Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20065614 1078 J. Southworth and J. V. Clausen: Absolute dimensions of the eclipsing binary DW Carinae Table 1. Identifications and astrophysical data for DW Car. T0 is a ref- The eclipsing nature of DW Car was discovered by erence time of mid-eclipse of the primary star. Hertzsprung (1924), who measured the orbital period to be 0.66382 d from a photographic light curve containing 251 ob- DW Car References servations. Van den Hoven van Genderen (1939) refined the Henry Draper Catalogue HDE 305543 1 orbital period using a 914-point photographic light curve, but Tycho number TYC 8957-1314-1 2 stated that the period should be doubled even though there was α2000 10 43 10.1 2 no clear evidence that adjacent minima had different depths. − δ2000 60 02 12 2 Gaposchkin (1952, 1953) published a photographic light curve Spectral type B1 V + B1 V 3 V 9.675 ± 0.005 4 containing over one thousand points and found an orbital period b − y 0.067 ± 0.005 4 of 1.3277504d. His estimated masses and radii are quite close to the values we measure below. m1 0.046 ± 0.008 4 c1 −0.017 ± 0.008 4 A spectroscopic orbit for DW Car was published by Ferrer β 2.528 ± 0.007 4 et al. (1985) based on 67 photographic observations. They found T0 (HJD) 2 446 828.6692(4) 4 no evidence for orbital eccentricity, and a systemic velocity Orbital period (days) 1.32774934(44) 4 which is consistent with membership of Cr 228. A spectral clas- + References: (1) Cannon (1925); (2) Høg et al. (2000); (3) Levato & sification of B1 V B1 V was provided by Levato & Malaroda Malaroda (1981); (4) Paper I; quantities in parentheses are uncertainties (1981). in the final digit of the quanities. We shall refer to the primary and secondary components of DW Car as star A and star B, respectively, where star A is eclipsed by star B at phase 0.0. Star A has a greater surface brightness and mass than star B. Secondly, there are several quantities which are not in general independently known for dEBs, including age, metal abundance and helium abundance, and these can be freely adjusted to help 1.2. Collinder 228 the models match the observed properties of a dEB. This last point means that it is extremely difficult to investigate the suc- The open cluster Cr 228 is a young and sparse cluster located cess of the treatment in theoretical models of several physical in the Carina spiral arm feature. Its immediate surroundings are phenomena, for example convective core overshooting, mixing quite nebulous, making photometric studies very difficult. length, mass loss and opacity (see Cassisi 2004, 2005). Feinstein et al. (1976) found a reddening of EB−V = 0.33 ± Extra constraints on the properties of a dEB can be obtained 0.08 mag, an age of less than 5 Myr and a distance modulus of − = ± if it is a member of a co-evolutionary group of stars (Southworth V0 MV 12.0 0.2 mag on the basis of a photoelectric UBV et al.